This invention relates to an art of decreasing power consumption of a power amplifier.
An amplification system for amplifying an audio signal amplifies a signal whose output level is small by a preamplifier and amplifies the signal amplified to a predetermined level by a power amplifier, thereby providing large output. Usually, the power amplifier mainly executes amplification of a signal to a level required for providing large output, and the preamplifier amplifies an audio signal to an input level required for the power amplifier and also executes sound field addition, tone control, volume control, etc.
On the other hand, the user who uses such an amplification system may not require large output using the power amplifier and may need only to obtain output from different loudspeakers using output from the preamplifier; the power amplifier involves large power consumption because of the characteristic of the power amplifier and there is a demand for stopping the function of the power amplifier and decreasing power consumption when the power amplifier is not used. To thus stop the function of the power amplifier, an art of turning off power supplied to the power amplifier is available. (For example, refer to JP-A-2004-104649)
However, to turn off the power supplied to the power amplifier as in the art disclosed in JP-A-2004-104649, a large load is imposed on a relay contact because of the magnitude of the supplied power and thus the reliability is degraded. On the other hand, if a relay is used upstream from a transformer to attempt to control at a stage before the supplied power becomes large, it is necessary to use a transformer for each of the preamplifier and the power amplifier and a very large space is required.